The invention relates to add/drop multiplexers, and more particularly to bi-directional line switched rings and their use of add/drop multiplexers.
Bellcore published a paper in December 1992 entitled xe2x80x9cAn Economic Feasibility Study for a Broadband Virtual Path SONET/ATM Self-Healing Ring Architecturexe2x80x9d in IEEE Journal on Selected Areas in Communication Vol 10 No.9 December 1992 on the economic feasibility of using ATM (asynchronous transfer mode) VP (virtual path) based technology to reduce the SONET ring cost of supporting the DS1 service, and a paper in September 1993 entitled xe2x80x9cCost-effective Network Evolutionxe2x80x9d IEEE Communications Magazine September 1993 on network evolution for broadband services based on the same principle. The two papers deal mainly on the economic viability of introducing ATM virtual path technology into the SONET (synchronous optical network) self-healing ring architecture. In general, in a typical SONET ring supporting add/drop multiplexers, the transport of data between any two nodes in the ring must follow some strict rules of configuration. For example, in a bi-directional line switched ring (BLSR), the key rules include that only half the number of tributaries of a fiber should be used for transport of data in any one direction. e.g. in an OC 48 BLSR, only 24 STS-1s (Synchronous Transport Signal, Level 1) are used in each fiber while the other 24 STS-1s are used for back-up in case of a fiber cut or node failure, and that data transport between any two nodes relies on a dedicated (or reserved) STS-1 or STS-3c (Synchronous Transport Signal, Level 3 concatenated) tributary between the nodes.
These rules result in the under-utilization of the potential bandwidth of the fiber, and introduce potential congestion in that each STS tributary can only support a single service. Furthermore, for the DS1 service, unless costly DS1 grooming is introduced at the DS1 level, there is a tremendous wastage of bandwidth.
In the above-identified papers, the Bellcore team describes the principle of transporting DS1 signals in the form of ATM cells within a STS-1 or STS-3c tributary terminating at two end nodes of a Fiber Ring network. Each pair of nodes in the fiber ring network is given a duplex VPI/VCI. Disadvantageously, cell processing is performed in each card and cell switching is also performed in each card, and a STS is reserved between nodes for each service subscribed following the existing BLSR procedure.
It is an object of the invention to obviate or mitigate one or more of the above identified disadvantages.
According to two broad aspects, the invention provides an add/drop multiplexer and multiplexing method. A plurality of STUs (service terminating units) are provided each having an associated identifier operable to receive information units from external services and to convert these into switching units containing a destination identifier, and to receive switching units having the associated identifier and combine them into information units and output them to the external services. A switching component is provided having a first set of dedicated receive ports operable to receive working channel traffic, a first set of dedicated transmit ports operable to transmit working channel traffic, a second set of dedicated receive ports operable to receive protection channel traffic, a second set of dedicated transmit ports operable to transmit transmit channel traffic, and an STU port for each STU operable to receive switching units from the respective STU and to send switching units to the respective STU. Switching units received by the switching component from the STUs are output through the first set of dedicated transmit ports unless a protection event has occurred affecting those ports in which case the switching units are output through the second set of dedicated transmit ports. The destination identifier of each switching unit received by the switching component from the first set of dedicated receive ports is examined by the switching component, and if the destination identifier is the same as the identifiers associated with one of the STUs, the switching unit is passed to the one of the STUs, and otherwise the switching unit is output through the first set of dedicated transmit ports unless a protection event has occurred effecting those ports in which case the switching unit is output through the second set of dedicated transmit ports.
Preferably, the first set of dedicated receive ports has a respective first subset of receive ports through which cells can be received from a first neighbour node, and a respective second subset of receive ports through which cells can be received from a second neighbour node. The second set of dedicated receive ports has a respective first subset of receive ports through which cells can be received from the first neighbour node, and a respective second subset of receive ports through which cells can be received from the second neighbour node. The first set of dedicated transmit ports has a respective first subset of transmit ports through which cells can be transmitted to the first neighbour node, and a respective second subset of transmit ports through which cells can be transmitted to the second neighbour node. The second set of dedicated transmit ports has a respective first subset of transmit ports through which cells can be transmitted to a first neighbour node, and a respective second subset of transmit ports through which cells can be transmitted to a second neighbour node.
Preferably, the multiplexer is connected to four fiber links, this consisting an incoming East-West fiber, an outgoing East-West fiber, an incoming West-East fiber and an outgoing West-East fiber. In this case, the ADM has a first fiber termination unit receive circuit connected to receive optical signals from the incoming West-East fiber, operable to convert these to electrical signals and to pass them to the first subset of the first set of dedicated receive ports and the first subset of the second set of dedicated receive ports. Furthermore, the ADM has a second fiber termination unit receive circuit connected to receive optical signals from the incoming East-West fiber, operable to convert these to electrical signals and to pass them to the second subset of the first set of dedicated receive ports and the second subset of the second set of dedicated receive ports. Furthermore, the ADM has a first fiber termination unit transmit circuit connected to receive electrical signals from the second subset of the first set of dedicated transmit ports and the second subset of the second set of dedicated transmit ports, operable to convert these to optical signals and to transmit them on the outgoing West-East fiber. Furthermore, the ADM has a second fiber termination unit transmit circuit connected to receive electrical signals from the first subset of the first set of dedicated transmit ports and the first subset of the second set of dedicated transmit ports, operable to convert these to optical signals and to transmit them on the outgoing East-West fiber.
Various techniques may be employed to select a particular port for a given switching unit. Preferably the next available port is selected.
A plurality of ADMs can be connected together by two fiber rings to form a bi-directional line switched ring.
Different traffic formats can preferably be handled such as DS3, IP, and Ethernet to name a few.
Preferably, the switching component is an ATM switch, the switching units are ATM cells, and the STU identifiers are ATM identifiers.